Climate Change Has Impacted Tree Growth in Temperate and Boreal Forests Since the Beginning of the 21st Century, a Meta‐Analysis Tells Us

Aim The first decade of the 21st century has marked a worsening of anthropogenic climate change which could induce a slowdown in tree growth. However, no consensus has emerged from studies published so far, as positive, negative, or no changes in long‐term tree growth in temperate and boreal forests were locally found. We conducted a meta‐analysis to reveal whether an overall change in tree growth trends has emerged in the last few decades and to determine which factors may have driven any such changes. Location Temperate and boreal forests worldwide. Time Period 1980–2009. Major Taxa Studied Angiosperm and gymnosperm tree species. Methods We performed statistical meta‐regression analyses on the growth of trees in 210 populations gleaned from 62 studies selected after a systematic review of more than 2300 published studies. We calculated effect size values at the population level to compare the slope of tree growth trends over time before and after a given year for each year between 1980 and 2009. We then statistically compared changes in growth among tree species, drought resistance classes, and the position of the species in their climatic niche. Results Estimated effect sizes were close to 0 throughout the end of the 20th century, then dropped from 2005 to 2008 and reached significant negative levels in 2008. Only drought resistance had a significant effect on the increased occurrence of negative effect size values. Main Conclusions Overall tree growth trends remained stable during the last two decades of the 20th century. However, a significant change in tree growth trends occurred during the first decade of the 21st century, with a negative trajectory. Europe most strongly displays this negative change in growth. Drought‐tolerant species were less negatively impacted.